Quantitative signal waveform measurement instruments such as oscilloscopes, spectrum analyzers, television waveform monitors, vector scopes, etc. utilize a cathode ray tube (CRT) for displaying the signal on a phosphor screen. A specially designed graticule scale or grid which is well suited to particular measurement applications is provided either internally or externally on the screen for quantitative measurement. Such graticule scales are selectively illuminated by one or more incandescent lamps provided at the edge of the CRT faceplate. The lamps are continuously controlled to provide constant or pulsed light output particularly adapted for different ambient light conditions, or for taking photographs of the displayed waveform in the single sweep mode of operation.
One typical prior art lamp control circuit disclosed in Japanese utility model publication No. 28690/72 assigned to the assignee of this invention is shown in FIG. 1 AC input voltage from AC power supply 10 is applied to the primary winding of transformer 12 and a low voltage induced in the secondary winding is rectified by diode 14 before being applied across potentiometer 20 and a series combination of the collector-emitter junction of transistor 16 and lamp 18. The controllable voltage on the wiper of potentiometer 20 is applied to the base of transistor 16. The emitter current of transistor 16 defines the lamp current of incandescent lamp 18. The lamp current is, of course, a function of the base bias current under control of potentiometer 20.
This prior art lamp control circuit is useful because no resistor is connected in series with lamp 18, thereby minimizing the power consumption and production cost. The lamp current may be controlled over a range from zero to a predetermined large current by using a low-power dissipation, less-expensive potentiometer 20. However, it is difficult to maintain the lamp current constant due to the base-to-emitter voltage (V.sub.BE), which is temperature dependent. In addition, there is no provision for switching the lamp current between zero and some certain value at a fast rate needed for trace photography.